Geodynamics

(geophysics) The branch of geophysics concerned with measuring, modeling, and interpreting the configuration and motion of the crust, mantle, and core of the earth.

The branch of geophysics that studies the processes leading to deformation of planetary mantle and crust and the related earthquakes and volcanism that shape the structure of the Earth and other planets. On the largest scale, these processes are a consequence of the transferof heat out of planetary interiors due to cooling at their surfaces. Rock contracts as it cools, so that its density increases. The cool surface layer is heavier than the interior and has a tendency to sink into it. At the same time, cooling and solidification of the metallic core heats the deepest portion of the surrounding rocky mantle, causing it to become buoyant. The resulting flow of the mantle causes deformation at the surface. Volcanism arises from the partial melting of hot mantle that rises toward the surface from the deeper interior, in response either to buoyancy or to surface deformation. Surface deformation also results from external loads, such as the distribution of ice and water, tidal loads due to the gravitational attraction of nearby planetary bodies, and meteor impacts. See also Earth, heat flow in; Geophysics; Volcano.

A planet's response to its internal heat flow depends largely on the rheology of deforming rock. At low temperatures, near the surface, rock behaves as a brittle-elastic material, allowing the propagation of seismic waves and the support of surface loads by elastic stresses. Deformation occurs by the formation of cracks or faults. On geologic time scales and at the higher temperatures of the deeper interior, thermally activated creep allows the solid, rocky mantle to flow like a viscous fluid. But even at these high temperatures, rock behaves elastically on short time scales so that elastic shear waves propagate through the slowly flowing mantle. Inthe case of the Earth in its current stage of evolution, plate tectonics describes how the surface behaves: large, cold, relatively rigid plates move laterally across the surface while the deeper mantle flows by creep. In the cold plates, deformation is largely confined to boundary faults between the plates. Faults slip with a stick-slip behavior, giving rise to large earthquakes that occur primarily on the plate boundaries. See also Earthquake; Plate tectonics; Rheology; Rock mechanics.

Given the difficulty of direct observation and the wide range of scales involved in phenomena of interest, multiple approaches are needed to understand geodynamic processes. Laboratory experiments on relatively small samples of rock are used to characterize the rock's physicalproperties, such as its rheology, at high pressures and temperatures. The rate of deformation due to creep in nature is much too slow to measure directly in the laboratory. Field studies of rocks once deep in the interior and brought to the surface by uplift and erosion provide evidence of the processes that have affected them. But the interior of the Earth where the processes of interest are actually occurring is not directly accessible for study. Thus geodynamicists must design large-scale observational experiments that allow them to create conceptual and physical images of the interior, using combinations of seismic, gravitational, electromagnetic, and heat-flow measurements. Variations in global gravity and corresponding surface topography can b remotely sensed from orbiting spacecraft. See also Earth crust; Earth interior; Geodesy; Geomagnetism; Seismology.